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My mom cut off her parents and I think one of the biggest things for her was sticking to her boundaries regardless of what others said. Granted, she had my dad at that point and got along with his family really well so maybe it helped to have their support in the matter. It was also well before the time of cell phones and social media which may have helped. Maybe start out with blocking your parents on social media or at least turn off notifications for any messaging services. Hide their posts so you can’t see what they’re saying. Make it clear you want nothing sent to your home and if they send you any mail, mark it “return to sender.”
I don’t think you have to explain yourself to the person who is guilt tripping you. You don’t owe them that. Maybe you can say that this is something that is not up for discussion with you and you’d appreciate it if they did not bring it up unless you specifically say something about it. If they don’t respect that and they refuse to change the subject if you ask them to, the conversation ends. You don’t deserve to put yourself through a potentially painful conversation or situation for the comfort of someone else.
Like I mentioned before, my mom had my dad’s family during this time. Do you have any people in your life who you consider to be family to you? It can even be a friend group, they don’t have to be blood relatives. Basically they’re the people you can go to when you’re feeling lonely or need support. You don’t even have to talk about your parents in that moment if you don’t want to. It can help just to know someone is there.